Massive Greenland iceberg calving

What massive Greenland iceberg calving looks like from the air

Jason Box

Jason Box,

2 September, 2015

Sometime May, 2014 AirZafari (+299 55 28 19) guest photographer Ruben Wernberg-Poulsen captured a new perspective on massive Greenland glacier calving.  In addition to the massive scale of the event seen clearly from the air, I think we’ve never seen that basal ice so clearly and so graphically from this birds eye perspective

The video is from none other than the site than that which  grabbed headlines as the world’s fastest glacier calved a giant area (12.4 sq km) and retreated (at least temporarily) to a new record minimum between 14 and 16 August, 2015 [1].

A rough dimension of the 2014 iceberg in the aerial video suggests a volume of ~180 million cubic meters of ice. If spread out over the Washington DC Mall from the Capitol steps to the Washington Monument (1.8 km x 0.4 km), this ice  would have a depth (~375 m) more than twice the 169 m height of the Washington Monument

Sapphire blue basal ice

Extreme pressure from 500-900 m ice overburden pressure plus extreme strain due to basal friction, strain heating seem to compress air bubbles that would make the ice appear white into blue ice. Else possible pressure melting, possible freeze-on of melt or sea water may also be at play to produce the curious sapphire blue basal ice on display in this extremely large iceberg calving event.

Commentary on the 12 sq km event 14 to 16 August, 2015

It’s impressive to see the Jakobshavn glacier retreat further, to a new record position upstream.

I wouldn’t say it’s the largest calving event to occur. The glacier lost a larger area between 2002 and 2004, a floating ice shelf. What’s different now: the ice is grounded or near floatation.

There is an interesting interplay to consider; accelerated forward motion of ice given loss of internal flow resistance on calving that will move the ice front forward quickly to replace the void. So, in not many days, the calving front may be back to the position as in the before image.

The calving front position represents the dynamic interplay between calving, producing front migration upstream and forward flow. Hypotherically, that dynamic could be in balance; no net front position change over time. Or the front could be in imbalance with retreat upstream as is the case in point.

This and most other Greenland glaciers are thinning (vertically), having the effect of un-grounding the ice from the bed at the glacier front. As this is a flooded, underwater system, these glaciers have because of buoyancy forces, what is called ‘marine instability’, allowing the glacier to retreat more quickly as they have fronts near or at floatation. Recent West Antarctic Ice Sheet glacier retreat (e.g. Rignot and others, 2014) suffers from the same marine instability.

This is an inherently unstable system given that the bed of the glacier is underwater several 10s of km upstream.

In this Greenland case, there is an upstream reverse bed slope, the bed gets deeper upstream at some point, not far from where the calving front has retreated to now. Further retreat upstream can be to deeper ice. This is unstable, so an even further retreat will occur given likely continued thinning.

A negative feedback here, something to dampen that instability, is an increase of ice flooding into the void from the sides of the flow, re-jamming it up a bit (regaining internal flow resistance), like partially recorking the flow.

There is a titanic struggle here between accelerated flow [due to loss of flow resistance from calving] and ice flooding into the void, partially re-jamming it up. Given likely continued thinning*, the winner is the loser, of more ice.

* from likely increased surface melting, likely increasing ocean temperatures. However, from year to year (the weather of climate), we may have a cold year that reverses this activity, leading to a short term advance, amid a longer term retreat observed at this an other Greenland glaciers in the past decade.

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Works Cited

• [1]massive calving event captured by the Arctic Sea Ice Blog.
• Rignot, E., J. Mouginot, M. Morlighem, H. Seroussi, and B. Scheuchl (2014), Widespread, rapid grounding line retreat of Pine Island, Thwaites, Smith, and Kohler glaciers, West Antarctica, from 1992 to 2011, Geophys. Res. Lett., 41, 3502–3509, doi:10.1002/2014GL060140.

Acknowledgements

Thanks Ruben Wernberg-Poulsen for capturing and sharing the video with World of Greenland. Thanks to Malik Milfeldt for the tip.